Topographic Maps Field Exercises

geostac@gmail.com

April 7, 2008

Map Scales

Individual topographic maps are commonly referred to as quadrangles (or quads), with the name of the quadrangle giving an idea of the amount of area covered by the map. The largest area covered by most topographic maps used for scientific mapping purposes (i.e. geologic mapping, habitat studies, etc.) are two degrees of longitude by one degree of latitude (see below).

A map of this size is referred to as a ‘two degree sheet’. One, two degree sheet can be divided into four smaller quadrangles, each covering one degree of longitude and 1/2 degree of latitude (‘one degree sheet’).

Each one degree sheet is subdivided into eight ‘fifteen minute quadrangles’, measuring fifteen minutes of latitude and longitude.

Finally, the smallest topographic quadrangle commonly published by the U.S. geological survey are 7.5 minute quadrangles, which measure 7.5 minutes of latitude and longitude. There are four 7.5 minute quads per fifteen minute quad, 32 per one degree sheet, and 128 per two degree sheet.

You can determine what type of quadrangle you are looking at by subtracting the longitude value printed in the upper (or lower) left corner of the map from the longitude printed in the upper (or lower) right corner of the map. This can also be done using latitude values, just remember that a two degree sheet only covers one degree of latitude and and one degree sheet only covers thirty minutes of latitude. This information is also commonly printed in the upper right hand corner of a map, under the title of the map.

The scale of a topographic map is here. In addition to a ratio scale, a bar scale is also shown to allow measurement of distances on the map and conversion to real-world distances.

As alluded to above, topographic (and other maps as well) come in a variety of scales. The scale of the map is determined by the amount of real-world area covered by the map. For example, 7.5 minute topographic quadrangles put out by the U.S. Geological Survey have a scale of 1:24,000. This type of scale is known as a ratio scale and what it means is that one inch on the map is equal to 24,000 inches (or 2000 ft) in the real world. Actually, it means that one of anything [cm, foot, etc.) on the map is equal to 24,000 of the same thing on the map. Another way of writing this would be a fractional scale of 1/24,000, meaning that objects on the map have been reduced to 1/24,000th of their original size.

Other map scales in common use for topographic maps are 1:62,500 (15 minute quadrangle), 1:100,000 (one degree sheet) and 1: 250,000 (2° sheet). The smaller the ratio is between distances on the map and distances in the real world, the smaller the scale of the map is said to be. In other words, a map with a scale of 1:250,000 is a smaller scale map than a 1:24,000 scale map, but it covers a larger real-world area.